U S. Pat. No. 2,891,968 describes the preparation of difluoromaleic anhydride by the dehydration of difluoromaleic acid, which in turn is obtained by the dehydrofluorination of trifluorosuccinic acid. The present invention represents an improvement over this preparation with respect to ease of synthesis of the difluoromaleic anhydride.
L. S. Kobrina, et. al., Zh. Org. Khim., vol. 8, pp. 2165-2167 (1972) obtained difluoromaleic acid (which can be converted to the anhydride, see U.S. Pat. No. 2,891,968, supra) in 54% yield by treatment of pentafluorophenol with peroxyacetic acid. Difluoromaleic acid was also obtained by analogous reactions of tetrafluoro-p- or tetrafluoro-o-benzoquinone, tetrafluoropyrocatechol and tetrafluoro-muconic acid. However, the preparation of this reference requires the oxidation of a fluorophenol, which is not necessary to the present procedure.
C. G. Krespan and D. A. Dixon, J. Org. Chem., vol. 51, pp. 4460-4466 (1986) describe the reaction of SO.sub.3 with perfluoro-2-butene. In this reaction only one of the terminal carbons reacted to form a carbonyl group, yielding the compound CF.sub.3 CF.dbd.CFCO.sub.2 SO.sub.2 F in relatively low yields even though high reaction temperatures and long reaction times were used. A small amount (0.05%) of trimethyl borate was present as a stabilizer in the SO.sub.3 used. The use of boron compounds as catalysts is discussed in this paper. However, neither difluoromaleic anhydride nor difluoromaleic acid are produced in the reviewed reactions.
Difluoromaleic anhydride is useful for the capping of polyimide prepolymers and polymers (U.S. Pat. No. 4,173,700), as a "cure site" monomer in fluoropolymers (U.S. Pat. Nos. 3,792,022 and 3,761,453) and as a comonomer with phenyl vinyl ether to form an alternating copolymer (G. Canessa C., Bol. Soc. Chil. Quim., vol. 26, pp. 25-37). In German Patent 2,649,743 it is used as a component of an antifouling coating. It is used as a chemical intermediate in the preparation of biologically active compounds in U.S. Pat. Nos. 3,485,829 and 3,485,827.
It is an object of the present invention to provide a one step process for the production of difluoromaleic anhydride from commonly available starting materials. A feature of the present invention is the production of useful intermediate compounds. It is an advantage of the present invention to provide catalysts for the reaction to facilitate commercial scale production of difluoromaleic anhydride. These and other objects, features and advantages of the present invention will become apparent upon having reference to the following description of the invention.